Transcript for George Zimmerman's Understanding of 'Stand Your Ground' Law

Now we turn to florida, george zimmerman on trial for the murder of trayvon martin. Today the jury heard from zimmerman's former college professor who seemed to catch zimmerman in a lie. Just what did he know about self-defense? Matt gutman has more from the florida courtroom. Reporter: It was a full-scale assault on george zimmerman's credibility. Do you remember zimmerman? I do. Reporter: The prosecution catching zimmerman in an apparent lie, asking zimmerman's former college law instructor -- you see george over here? How you doing, george? Reporter: If the defendant knew anything about florida's law allowing someone to use deadly force if mortally threatened. Did you discuss specifically self-defense and stand your ground laws and the connection of violent crimes such as murder? Yes. Reporter: It was only yesterday that jurors watched zimmerman saying the exact opposite in a fox news interview. Prior to this night, this incident, had you even heard of stand your ground? No, sir. You have never heard about ti before? No. Reporter: But zimmerman actually got an "a" in that class in 2010. His team arguing this all comes down to self-defense. You don't have to wait until you're almost dead before you can defend yourself. No, I would advise you probably don't do that. Reporter: Eliciting a chuckle from zimmerman -- Reporter: That smile quickly faded. George zimmerman being hammered away his credibility by the prosecution. Only one person who was there for the start of the fight and the shooting. That was george zimmerman. In order to win the prosecution has to ensure the jury doubts his version of events. Now we turn to arizona where tonight 129 homes have been

This transcript has been automatically generated and may not be 100% accurate.

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